Air bubble detector

ABSTRACT

Air bubbles may be characterized by an air bubble detector by choosing an optimum set of frequencies and then comparing a return signal from a sensor receiving those frequencies against an internal reference. The number of pulses that exceed the internal reference represents a width and may be counted. The width, as counted, may be correlated to bubble characteristics including volume.

PRIORITY

The present application claims the benefit of U.S. ProvisionalApplication Ser. No. 61/150,706, filed Feb. 6, 2009 which is hereinincorporated by reference in its entirety.

THE FIELD OF THE INVENTION

The present invention relates to air bubble detectors. Morespecifically, the present invention relates to an air bubble detectorand related control circuitry with improved sensing and reporting of airbubbles.

BACKGROUND

Bubble detectors have become an important safety feature in manyapplications including industrial and medical applications. For example,in medical applications, if air is introduced into the bloodstream viaan infusion tube, an air embolism may occur. The effects of an airembolism may range from little or no effect to death, typicallydepending on the amount of air which is introduced. Industrially, it isoften desirable to detect the presence of air in a fluid stream to moreaccurately dose the fluid or to avoid introduction of air into a system.Therefore, it is often useful to automatically detect bubbles.

In the medical field, air bubble detection is often important. Typicalapplications include dialysis, enteral feeding, and intravenousintroduction of fluids. In these situations, the greater the volume ofair, the greater could be the risk of harm.

Current bubble detectors send ultrasonic signals through a fluid todetectors and measure amplitude changes on the received signal. Theoptimum ultrasonic frequency for detection is often different fordifferent situations, as the particular detector, tubing, or fluid mayaffect the transmission of the signals. Thus, some bubble detectorssweep the entire possible spectrum of ultrasonic frequencies on eachpass to reduce the risk of failure in bubble detection. Sweeping theentire spectrum typically results in time spent scanning for bubblesusing less than optimal frequencies. The optimum frequency for bubbledetection may be affected by piezoelectric crystal proportion,composition, construction tolerances and dynamic factors such astemperature, tubing composition, fluid composition, and the couplingbetween the sensor and the tube.

False alarms have become a problem in bubble detection. These can becaused by microbubbles that stick to a wall, bubbles that oscillate backand forth in front of a sensor, the decoupling of tubing, etc. Falsealarms may decrease the trust in the system and increase the workload ofstaff, and may cause problems or delays in dosing of medication or thelike.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved airbubble detector.

According to one aspect of the invention, an air bubble detector isprovided which sweeps a frequency range and identifies an optimumfrequency which is used to obtain a more accurate characterization ofbubbles in the fluid or the volume of air which has passed through thedetector.

According to another aspect of the invention, an air bubble detector isprovided which measures the amount of time a sensor signal is above athreshold, and which uses the threshold measurement to characterize theair bubble which caused the signal response.

According to another aspect of the invention, a series of bubbledetectors are provided which may be used in combination to determinedirection and speed of bubbles in the fluid.

These and other aspects of the present invention are realized in an airbubble detector as shown and described in the following figures andrelated description.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention are shown and described inreference to the numbered drawings wherein:

FIG. 1 shows a functional diagram of an air bubble detector.

FIG. 2 shows a flow diagram of optimal frequency detection in an airbubble detector.

FIG. 3 shows a flow diagram of optimal frequency detection with anincreased repetition rate in an air bubble detector.

FIG. 4 shows a flow diagram of optimal frequency detection with abattery saving sleep state in an air bubble detector.

FIG. 5 shows a flow diagram of recalibrating optimal frequency detectionin an air bubble detector.

FIG. 6 shows a flow diagram of bubble characterization throughmeasurement of time.

FIG. 7 shows the flow diagram of FIG. 6 broken down into more specificprocesses.

FIG. 8 shows a comparison between an expected signal and a receivedsignal.

It will be appreciated that the drawings are illustrative and notlimiting of the scope of the invention which is defined by the appendedclaims. The embodiments shown accomplish various aspects and objects ofthe invention. It is appreciated that it is not possible to clearly showeach element and aspect of the invention in a single figure, and assuch, multiple figures are presented to separately illustrate thevarious details of the invention in greater clarity. Similarly, notevery embodiment need accomplish all advantages of the presentinvention.

DETAILED DESCRIPTION

The invention and accompanying drawings will now be discussed inreference to the numerals provided therein so as to enable one skilledin the art to practice the present invention. The drawings anddescriptions are exemplary of various aspects of the invention and arenot intended to narrow the scope of the appended claims.

Turning now to FIG. 1, a functional diagram of an air bubble detectorsystem 10 is shown. The detector 10 includes a microcontroller 20 and asensor circuit 30. The microcontroller 20 determines the optimum rangeof scanning frequencies for the detector and operates the detector usingthis range of frequencies. When operating the detector 10, thecontroller 20 determines optimal frequencies for scanning and performsscans, recording the duration or width of a signal received whichexceeds a predetermined threshold. The controller 20 may periodicallyscan the entire frequency to update the optimum frequency range used forscanning. The microcontroller 20 sends a signal to the sensor circuit 30indicating a broad set of test frequencies desired for calibration. Thesensor circuit 30 sends back a return signal to the microcontrollerbased on the transmission of a test frequency range by a sensor througha fluid path 33. An optimum set of operational frequencies is chosenfrom the test frequencies. Once an optimum range of frequencies ischosen, the microcontroller sends the optimum set of frequencies to thesensor circuit 30 and monitors the return signal from the circuit. Foreach return signal above an internal threshold value, one is added to awidth counter. Based on the width count during the optimum set offrequencies, the microcontroller 20 determines if bubbles are presentand characterizes any bubbles in the fluid path 33. Once the optimum setof frequencies have been sent, the width counter is reset and theoptimum set of frequencies are resent or the calibration repeated.

In one embodiment, the microcontroller 20 controls the frequencies usedfor scanning by sending a pulse width modulated (PWM) voltage ramp 32 tothe sensor circuit 30. The PWM voltage ramp 32 may direct a voltagecontrolled oscillator (VCO) 34 to sweep a set spectrum of frequencies.The resulting waveform may be passed through an output driver 35 toisolate the VCO and provide the necessary voltage and current to thetransmitter 36, while maintaining waveform fidelity. The transmitter 36,typically a piezoelectric element, receives the electrical waveformsignal, and converts it to energy such as ultrasonic waves which aretransmissible through the fluid path 33. The receiver 37 detects thetransmission of the energy from the transmitter 36 and converts thetransmission into a return signal acceptable by the microcontroller 20.According to a preferred embodiment of the invention, the air bubbletransmitter 36 and receiver 37 are piezoelectric elements usingultrasonic signals to detect air bubbles.

Inside the microcontroller 20 a comparator 22 receives the signal fromthe sensor circuit 30 and compares it with an internal referencethreshold value. If the signal exceeds the internal reference, one countis added to the width counter. After the sweep of the set optimumspectrum of frequencies, the width counter may be compared to anexpected value. This comparison may be correlated with the results ofknown bubble sizes. The result of the comparison may be output to otherdevices connected to a digital output 23 or analog output 24. As such,the system 10 can output signal indicative of the size and quantity ofthe bubble rather than simply outputting receiver pulses.

In one embodiment, the bubble detector is connected with a pump such asan enteral feeding pump. The fluid path 33 consists of a silicone tubewith a solution therein. The tube is captured between the transmitterand receiver through friction or a clamping mechanism that fitspartially or wholly around the tube without damaging it. Thetransmitter/receiver pair typically operates on a frequency range from1.7 MHz to 3 MHz, with a center (optimum) frequency dependent on aspecific installation and environment. The frequency range may varydepending on the design of the transducer. For example, many transducerswill use the above range, with a resonant frequency of 2 MHz. However,some transducers may have a different resonant frequency such as 3 MHz,such that the sweep range would be shifted up to accommodate the higherresonant frequency. The air bubble detector may be connected to an alarmthat may signal a pump, or staff that a bubble has occurred and thataction should be taken in response to the bubble. The air bubbledetector 10 may also be connected to an information gathering device. Inanother embodiment, the bubble detector may be connected with anindustrial fluid carrying tubing or line and used to monitor bubbles inthe fluid as discussed.

Turning now to FIG. 2, the process of calibrating an air bubble sensor40 is shown. Once the air bubble sensor detects an on state 42 or astart signal, a calibration signal 43 is sent to the receiver. Thecalibration signal 43 includes a sweep of potential ultrasonicfrequencies which is used by the sensor 40 to detect air bubbles. Thecontroller circuit 20 creates a ramped signal using a pulse widthmodulated signal. The ramp signal 32 is sent to a voltage controlledoscillator 34 which converts the voltage ramp into an oscillationsignal. The output driver 35 amplifies the signal and transmits thesignal to the piezoelectric transmitter 36. Thus, the transmitter 36transmits a broad range of ultrasonic signals which encompasses thepotential operating range for the transmitter 36, receiver 37 andphysical system (such as the tube 33 and other physical structures). Theoptimum frequency is detected 44 by measuring the amplitude of thereceived signals at the different frequencies and detecting the peakamplitude or amplitudes which correspond to the resonant frequencies forthe system. The optimal frequency or frequencies are stored 45 for lateruse in operating the bubble detector.

The system 10 selects a narrowed range of frequencies for use inscanning for bubbles based on the optimal frequencies. Typically, apredetermined range of frequencies centered around the optimal frequencyis used as a set of scanning frequencies. The narrowed range offrequencies is beneficial as it provides increased efficiency andspecificity for bubble size, since the scanning is performed using thefrequencies which correspond to the resonant frequencies of the system.The use of the narrowed range of frequencies in scanning also increasesresolution in the time domain because, for a given sweep rate throughthe frequency range, the time to scan is less for a narrower range offrequencies. This allows the scan to be repeated quicker, providing afaster scan rate. The measured width of a received signal above athreshold provides information about the bubble size as well asinformation about the degree of coupling between the tube and thesensor.

The calibration of the sensors during the use of the air bubble sensormay allow for a narrower band of frequencies to be used. The calibrationmay allow for an optimum frequency to be chosen based on theenvironmental effects upon the signal transmission. Transmissionfrequency response may depend on variables that include temperature,transmitter composition and geometry, receiver composition and geometry,fluid path walls, fluid, bubble composition and output strength. Sincevarious conditions such as temperature or fluid composition may changeduring the use of the device 10, the detector system 10 is oftenoperated by selecting an optimal detection frequency, scanning for apredetermined period of time using the optimum frequency, and thenupdating the optimal frequency. The optimum frequency is updated byperforming a full sweep of the ultrasonic frequency range for the deviceand selecting new optimal frequencies. The system 10 would not performthis update while a bubble is being detected, as this could interferewith the ability to detect the optimal frequency.

Turning now to FIG. 3, the calibration and use of the air bubble sensor10 is shown. The air bubble sensor is calibrated as discussed before bymeasuring the response to a full frequency signal sweep, and then usingthe optimum frequencies to narrow the frequency sweep to send a shorterdetection signal, as indicated by box 46. The signal is received andinterpreted in box 47 faster because the smaller frequency sweeprequires less time and allows the data collection and processing tocomplete faster. Because the frequency sweep is narrowed, the sweep maytake less time and the send detection signal step in box 46 andinterpret received signal step in box 47 may complete faster. As thosesteps complete faster, the air bubble detector may detect bubblemovement at a higher response rate that allows for accurate readings athigher flow rates within the fluid path than conventional sensors.

The output signal from the receiver 37 is processed by comparison to athreshold value in comparator 22. The threshold value used may bedetermined during the calibration of the signal frequencies used forscanning. As discussed, a narrowed scanning frequency range isdetermined by selecting the receiver signal frequencies with the largestvalues (i.e. the sympathetic or resonant frequencies) and utilizing apredetermined frequency range around these frequencies. When this testis being performed, baseline values for the frequency range may berecorded and used as threshold values. The signals from the receiver 37are compared to the threshold values to detect a bubble. A bubble hasbeen discussed herein as creating a signal above the threshold value. Itwill be appreciated that, depending on how the receiver element 37 isreferenced and how the signal is transmitted to the comparator 22, abubble may cause a signal which is either above or below the thresholdvalue. Thus, the term ‘above a reference value’ is used as a convenientway to refer to signals which deviate from the reference value.

In use, the deviation from the reference signal is monitored and whenthe deviation exceeds a predetermined value, it is determined that abubble is present. The comparator determines when a bubble is present bydetecting the signals which exceed an allowed deviation from thereference value. The timer 21 shown in FIG. 1 may both control thescanning and operation of the ramp and signal generation circuits aswell as the functioning of the comparator. The timer 21 also may performthe function of the counter which counts the time width of the bubblesignal. That is to say that, for each time unit where the receiversignal exceeds the reference value and indicates a bubble, the counteradds a count to the count total. The count total indicates the totalsize of the bubble, and is thus used to characterize the bubble. Whenthe bubble passes and the receiver signal returns to the thresholdvalue, the counter is reset.

Turning now to FIG. 4, a method of using the air bubble detector system10 is shown that may result in decreased power usage. The air bubblesensor is calibrated as discussed above. The optimum frequencies areused to narrow the frequency sweep used for detection of bubbles,resulting in a detection signal with a shorter time duration as seen inbox 46. The signal is received and processed as indicated in box 47.Instead of immediately sending the next detection signal 46, the airbubble detector may enter a powered down state or sleep state as seen inbox 48. Because the detector 10 operates using a narrow band of optimumscanning frequencies, the time to complete a single scan may bemicroseconds. The time duration where a bubble may be between thetransmitter 36 and receiver 37, however, may be fractions of a second.Thus, the detector may complete a scanning cycle in a few millisecondsor less, power down for a hundred milliseconds, and then perform thenext scan cycle. The powered down state may require less power for theinactive cycle time, resulting in a more efficient use of power and evendecreased power requirements.

The decreased power usage is useful in situations that require batterypower or sensitivity to the transmitted energy. As the optimum set offrequencies is merely a selection of frequencies of the total possiblefrequencies, the air bubble detector may only cause periodictransmissions as required by the flow rate within the fluid path. Thusthe battery drain is reduced.

Communication may exist between the air bubble detector and a pump whichis driving the flow within the fluid path. The repetition rate of theair bubble detector is influenced or controlled by the pump. Thus, thepump may operate the detector system 10 such that, as the flow rateincreases, the scan repetition rate may increase as well. As the flowdecreases, the air bubble detector may operate at a slower scanrepetition rate.

Turning now to FIG. 5, it is shown how the system 10 may operate toperiodically calibrate the air bubble sensor. The initial calibrationand detection proceed as before. After interpreting the received signalin box 47, the air bubble detector determines if a recalibration isrequired in box 49. This recalibration is determined by a pre-specifiedtime period, changes noted in the environment, degradation of thereceived signal or external request from a user. The recalibration mayallow the air bubble detector to adapt to changing environmentalconditions while maintaining its enhanced performance.

Turning now to FIG. 6, the process of detection of an air bubble 50 bythe air bubble detector is shown. The air bubble detector begins byresetting its state in box 51 to prepare for a new set of measurements.As a measurement is taken in box 52, the measurement is compared againstan internal reference in box 53. The air bubble detector stores thisinformation about whether each comparison exceeds the internalreference. When enough measurements have been taken, the air bubbledetector will use the measurements to characterize any bubble detectedin box 55 and output the results in box 56. The system can track longterm changes in the received signal and thus adjust for the signalthreshold used to detect bubbles or recalibrate the frequency range usedto detect bubbles. The short term changes in the received signal areused to detect bubbles.

One advantage of storing information related to whether a set ofmeasurements exceeds an internal reference is a decreased sensitivity toenvironmental effects causing changes in amplitude. Instead of measuringa potentially noisy signal amplitude, the number of times the signalpasses a preset standard is recorded. This duration measurement, orwidth measurement is correlated to bubble size. Using the volumecalculation, the volume of a series of bubbles is added together for atotal volume measurement. The microcontroller may then set a limit onbubble size, total air volume, volume within a time period, or acombination depending on the application.

The output of the air bubble detector is digital, analog, data or anycombination of these. The microcontroller may use a PWM driver ordigital to analog converter to generate an analog voltage or currentoutput signal. The analog output could convey to the host system thevarious bubble sizes that are detectable by the sensor. The digitaloutput may transmit similar or more complete data to a host system usingany number of protocols including SPI, I2C or others.

Turning now to FIG. 7, the process of FIG. 6 as broken down into morespecific processes is shown. The same process of resetting state in box51, taking a measurement in box 52, comparing the measurement in box 53,determining if the measurements are complete in box 54, andcharacterizing the bubble in box 55 are shown with more incrementalsteps.

By resetting its state in box 51 the air bubble detector may prepare totake new measurements. In one embodiment, the reset state in box 51 isperformed along with determining the internal threshold in box 62 thatmay result in a return signal being counted as a bubble in the widthmeasurement. The frequencies to be swept setting is reset to start atthe low frequency in box 63. The width counter may also be reset in box64.

Small bubbles are detected based on the width of a received signal at athreshold signal level. Long term changes in the width of the signal atthe threshold level is used to adjust the bubble size estimate. To beginaccumulating for large bubbles which would result in several receivedsignals with a signal level above the threshold, the initial signalwidth could serve as a trigger to start accumulating multiple receivedsignals together to determine the size of a large bubble.

The internal threshold may act as a barrier to noise. If the internalthreshold is too low, noise may cause a small signal amplitude to becounted as a bubble, producing a false width. Similarly, if the internalthreshold is too high, the return signal may not have enough amplitudeto exceed the internal threshold and bubbles may not be counted.Therefore an internal threshold is selected above the noise but lessthan the return signal amplitude to avoid these problems.

After resetting state in box 51, the air bubble detector may takemeasurements in box 52. In one embodiment, the air bubble detector maytransmit a wave through the fluid path in box 65. The wave is detectedin box 66 and sent to be compared with an internal threshold in box 67.

The transmitter/receiver pair may differ somewhat depending on the fluidpath, and typically is an ultrasonic transmitter and receiver, such as apair of piezoelectric elements. The specific choice of technology maydepend on the ability of the chosen sensor pair to penetrate differentparts of the fluid path. Such considerations may include the fluid pathwalls, the fluid itself and the bubbles within.

After taking measurements as seen in box 52, the air bubble detector maycompare the measurements in box 53. In one embodiment, the measurementsis compared with an internal threshold in box 67. The air bubbledetector may compute whether the return signal exceeds the internalthreshold in box 68. If the return signal does exceed the threshold, oneis added to the width counter in box 69 and then processing may continuein box 70. If the return signal does not exceed the threshold,processing may move to box 70.

After comparing the measurements in box 53, the air bubble detector maydecide whether the measurements are complete in box 54. If, in box 70,it is decided that the measurements are not complete, the air bubbledetector may move to the next frequency in box 71 and then return to box65 to send the next wave. If the test cycle is complete, the air bubbledetector will move to box 55 to use the information collected from themeasurements.

The decision on whether to complete the test cycle in box 70 may includethe end of a frequency sweep, detected failure in the system or enoughdata to characterize the bubble. In one embodiment, the air bubbledetector may repeatedly sweep a frequency spectrum. The test cycle iscomplete when the frequency spectrum has been swept. In anotherembodiment, a detected failure in the system may cause the test cycle tocomplete and then reset. In some systems this wait to reset is requiredbecause an output is expected at certain intervals. An immediate resetmay not be possible because a watchdog timer may cause an undesirablefull system reset due to a missed output timing. In another embodiment,sufficient data may exist to characterize a bubble, such as a totalocclusion of the fluid path by the bubble, and a full sweep is notrequired.

After the decision in box 54 results in the cycle being complete, theair bubble detector may characterize the bubble in box 55. The receivedsignal is proportional to the width counter. Thus, the width measurementmay contain information related to signal strength. In one embodiment,the stronger the received signal, the greater the width is. As a bubblemoves through the sensor the signal strength may decrease that may alsobe manifested in the width decreasing. A bubble size is correlated witha width much like an existing sensor correlates amplitude to bubblesize.

In one embodiment, the air bubble detector is connected in line with anintravenous infusion line. The tube is captured between the transmitterand receiver through friction or a clamping mechanism that fitspartially or wholly around the tube without damaging it. Thetransmitter/receiver pair may operate on an acoustical frequency from1.7 MHz to 3 MHz. As a bubble moves through the sensor the signalstrength may decrease, which causes the measured width to decrease.Should the measured width decrease enough, an alarm is alerted to theproblem.

While the processes described above may appear linear in thisdescription for ease of understanding, the actual steps are performed inparallel. For example, in one embodiment, the characterization of thebubble may work in parallel with the next detection of bubbles as seenin boxes 51-54. This will allow for a further gain in repetition rate.Thus, the tasks described may run in parallel with the tasks describedor with other tasks not included in this disclosure.

While the specific embodiments described may use a single sweep offrequencies to determine bubble presence, multiple sweeps of thefrequencies are used to characterize a bubble or set of bubbles. Thewidth is stored in multiple counters that are used to perform a finaldetermination of bubble volume. In one embodiment, the width countersare used on a rolling basis to average out the effects of noise.

In another embodiment, multiple sensors are attached to the fluid pathallowing the air bubble detector to detect bubble flow direction, speedand flow rate. The different sensors are placed upstream/downstream fromeach other, and may thus detect the bubble as it flows through thetubing. The information gathered from multiple sensors may also preventalarms from microbubbles that stick to a wall, sticky bubbles, andbubbles that oscillate back and forth in front of a sensor. Since, inthese cases, only one sensor would detect the bubble (at least for aperiod of time before the bubble eventually moves), the system wouldidentify this as a single bubble and not many different bubbles,preventing false alarms due to an incorrectly high bubble count. Thealarm is triggered by a bubble that has been characterized by one sensorand then sensed by another. Often, bubbles or the like which are notmoving towards the patient through the tube should not trigger the alarmor stop the fluid flow. In the event that the bubble does begin to movetowards the patient, the bubble would be counted.

In another embodiment, the air bubble sensor may communicate with anexternal device that measures or knows the flow rate of the liquid. Theair bubble sensor may adjust its repetition rate in accordance with theinformation given by the external device. Such external devices mayinclude a pump, flow sensor or manual input. This communication may alsoprevent alarms from the decoupling of tubing as sensed by the externaldevice.

Turning now to FIG. 8, a reference signal 82 compared with a receivedsignal 83 is shown. The air bubble detector may measure the receivedsignal by the number of counts crossing the voltage reference threshold84 which it reads during a frequency sweep. The measurement may resultin the width 85 of the received signal with respect to the internalvoltage reference setting 84 and reference signal 82 for a known bubblesize, tubing set, and loading conditions. The received signal envelope86 may then be proportional to the reference signal 82. Thisproportionality may result in the width measurement containinginformation related to signal strength or bubble size.

It may also be appreciated that the initial selection of optimumfrequencies may provide advantages to characterizing the bubble. As aresult of the frequency selection and repetition rate, bubbles are moreaccurately measured by the width counter. In fact, one advantage mayinclude a more granular volume result due to an increased speed of themeasurements.

The signal shown and discussed with respect to FIG. 8 can also be usedto determine if a tube is properly loaded in the sensor, and thus in thepump or device. The receive signal amplitude is at low level if notubing is loaded in the sensor, at a high level if fluid-filled tubingis loaded in the sensor, and at an intermediate level (typically closeto but distinguishable from the low signal level) for tubing which isloaded in the sensor but which has a bubble present within it.

There is thus disclosed an improved air bubble detector. It will beappreciated that numerous changes is made to the present inventionwithout departing from the scope of the claims.

1. A bubble detection system comprising: a fluid path; a piezoelectrictransmitter and a piezoelectric receiver disposed adjacent the fluidpath for detecting bubbles therein; control circuitry for operating thetransmitter and receiver comprising: a circuit for generating a widerange frequency sweep in the transmitter; a circuit for analyzing thereceiver output and determining a resonant frequency for thepiezoelectric transmitter and receiver and fluid path; memory forstoring the resonant frequency; and a circuit for operating thetransmitter at a reduced frequency range at the resonant frequency inorder to thereby detect bubbles in the fluid path.
 2. The system ofclaim 1, further comprising: a comparator circuit for comparing thereceiver signal while detecting bubbles to a signal threshold todetermine if bubbles are present.
 3. The system of claim 2, wherein thecomparator circuit determines the time period for which a signal exceedsthe signal threshold and determines bubble size based on the timeperiod.
 4. The system of claim 1, wherein the circuit for generating awide range frequency sweep in the transmitter comprises a voltage rampgenerating circuit and a voltage driven frequency generation circuit. 5.The system of claim 1, wherein the fluid path comprises a tube fordelivering a liquid.
 6. The system of claim 1, wherein the circuit foranalyzing the receiver out and determining a resonant frequency for thepiezoelectric transmitter and receiver and fluid path comprises acomparator circuit.
 7. A bubble detection system comprising: a fluidpath; a piezoelectric sensor coupled with the fluid path and configuredfor detection of bubbles in the fluid path; an internal receiver signalreference; and a comparator circuit configured to detect bubbles anddetermine bubble size within the fluid path by determining the length oftime an output signal of the sensor exceeds the internal reference. 8.The system of claim 7, further comprising: an input signal sourceconfigured for sweeping a broad range ultrasonic signal through thesensor; a circuit for analyzing the received signal from the broad rangeultrasonic signal to thereby determine a sympathetic frequency foroperation of the sensor; a circuit for storing the sympatheticfrequency; a circuit for operating the sensor at a narrow rangeultrasonic frequency surrounding the sympathetic frequency;
 9. Thesystem of claim 7 wherein the output signal is comprised of pulses andthe comparator circuit is configured to determine bubble size bycounting the number of pulses of the output signal of the sensor thatexceed the internal reference.
 10. A method of detecting bubblescomprising: sending an ultrasonic signal through a fluid path by anultrasonic transmitter; reporting a detected signal by an ultrasonicreceiver; selecting an internal receiver signal reference; determiningthe amount of time the detected signal exceeds the internal reference;and calculating bubble size based on the amount of time the detectedsignal exceeds the internal reference.
 11. A method of detecting bubblesin a fluid path comprising: transmitting a broad frequency range ofultrasonic signals through a fluid path; receiving the broad rangeultrasonic signals after transmission through the fluid path;determining a sympathetic frequency for the fluid path based on thestrength of the received broad range signals; storing the sympatheticfrequency; transmitting a narrow frequency range of ultrasonic signalssurrounding the sympathetic frequency through the fluid path to detectbubbles; and receiving the narrow range ultrasonic signals to detectbubbles.
 12. The method of claim 11, further comprising: for a period oftime, repeatedly transmitting the narrow range ultrasonic signalsthrough the fluid path to detect bubbles; and transmitting a broadfrequency range of ultrasonic signals through a fluid path; receivingthe broad range ultrasonic signals after transmission through the fluidpath; determining a new sympathetic frequency for the fluid path basedon the strength of the received broad range signals; storing the newsympathetic frequency.
 13. The method of claim 11, wherein the methodcomprises; storing an internal threshold received signal level;comparing the received narrow range ultrasonic signals to the thresholdto determine when the received signals exceed the threshold to therebydetermine the presence of a bubble.
 14. The method of claim 13, whereinthe method comprises: determining the time duration when the receivedsignals exceed the threshold and calculating the bubble size based onthe time duration.
 15. The method of claim 14, wherein the receivedsignals are separate signal pulses, and wherein the method comprisescounting the number of times the signal pulses exceed the threshold tothereby determine the bubble size.
 16. The method of claim 11, whereinthe method comprises: for a period of time, repeatedly transmitting thenarrow frequency range of ultrasonic signals surrounding the sympatheticfrequency through the fluid path; receiving the narrow range ultrasonicsignals to detect bubbles; after a predetermined event, transmitting abroad frequency range of ultrasonic signals through a fluid path;receiving the broad range ultrasonic signals after transmission throughthe fluid path; determining a new sympathetic frequency for the fluidpath based on the strength of the received broad range signals; andstoring the new sympathetic frequency.
 17. The method of claim 16,wherein the predetermined event is a change in physical conditions inthe fluid path.
 18. The method of claim 16, wherein the predeterminedevent is the passage of a set time period.